3/4*2又2/5*2
=3/4*2*12/5
=3/2*12/5
=18/5
=3.6
7/11-8/7*5/8+5/14
=7/11-（5/7-5/14）
=7/11-7/14
=21/154
（1/3*6/7-1/14）÷4/15
=（2/7-1/14）÷4/15
=3/14×15/4
=45/56